The issues #7022, #7023, #7024 demonstrate a similar set of issues where a module is absent from modules, but is present in some other module list. Dune unfortunately ignores this discrepancy. At best, this is a harmless mistake, and at worst, this leads to some confusing build errors.

I propose that we do the following:

1. We warn the user when this mistake is made for now.
2. In 3.8, we turn this into a hard error.

Wdyt @nojb @emillon